The term “battery” originally meant a plurality of electrochemical cells connected in series. Nowadays however, individual electrochemical cells (individual cells) are often called batteries. When an electrochemical cell is discharged, a chemical reaction which provides energy takes place, the reaction being made up of two partial reactions which are electrically coupled to one another but are physically separate from one another. Electrons are liberated at the negative electrode in an oxidation process, this resulting in a flow of electrons across an external load to the positive electrode which takes up a corresponding quantity of electrons. Therefore, a reduction process takes place at the positive electrode. At the same time, an ion current which corresponds to the electrode reaction occurs within the cell. This ion current is ensured by an electrolyte which conducts ions. The discharging reaction is reversible in secondary cells and batteries, that is to say it is possible to reverse the conversion of chemical energy into electrical energy which took place during discharging.
From amongst the known secondary cells, comparatively high energy densities are achieved, in particular by lithium-ion cells, that is to say by cells in which lithium ions migrate from one electrode to the other during charging and discharging processes. Cells of this kind are particularly suitable for use in portable devices such as mobile telephones and notebooks. However, they are also of particular interest as energy sources for motor vehicles.
Cells of lithium-ion batteries generally have combustible components. For example, the electrolyte of a lithium-ion cell often comprises an organic solvent such as ethylene carbonate, for example, as the main component. In conjunction with high energy densities of cells of this kind, this constitutes a potential hazard which should not be underestimated. It is therefore necessary to take particular safety precautions to be able to preclude risks for users or at least to keep the risks as slight as possible.
Lithium-ion cells may enter a critical state in which there is a risk of fire under certain circumstances, particularly when they are mechanically damaged or as a result of being excessively charged. Excessive charging of a lithium-ion cell can lead to deposition of metallic lithium on the surface of the negative electrode and also possibly to destruction of the electrolyte contained in the cell. The latter may lead to severe gassing of the cell. In extreme cases, this leads to damage to a housing surrounding the cell. As a result, moisture and oxygen can enter the cell and this can result in explosive combustion.
To avoid this, it is usual to provide lithium-ion cells with safety devices. A suitable circuit arrangement to electronically monitor the operational safety of rechargeable lithium-ion cells is known, for example, from DE 101 04 981 A1. The use of fuses to increase the safety of lithium-ion batteries is known from DE 10 2008 020 912 A1. DE 10 2007 020 905 A1 discloses cells having a discharge conductor arranged on a thin plastic film and having a predetermined breaking point. If the film deforms, for example, as a result of cell gassing, the discharge conductor is destroyed at the predetermined breaking point, as a result of which the cell is irreversibly and permanently deactivated.
Both fuses and safety means in which an electrical contact is destroyed or interrupted are known to be very reliable. However, one problem is that the battery initially remains in a critical, possibly also dangerous, state after safety devices of this kind are tripped. Furthermore, since the safety devices are irreversibly tripped, it is generally not possible to actively change something in this state.
It could therefore be helpful to provide batteries, in particular lithium-ion batteries, in which a reliable and simple safety solution which takes the cited problems into account is realized.